comms/emacs-eat: Ensure Emacs version matches byte-compiled elisp

Change USES=emacs:build to USES=emacs. This creates separate flavors
with a run-time dependency on the corresponding version of editors/emacs
or editors/emacs-devel. As a result, elisp is compiled and run under
the same Emacs version, avoiding potential subtle breakage caused by
version mismatches in byte-compiled files.

While here, fix a typo in pkg-descr.

PR: 290546
Sponsored by: The FreeBSD Foundation